In optical transmission systems, cables, such as fiber-optic cables, are used to transmit information. In some systems, the cables extend from an optical line terminal (“OLT”) or other optical device to one or more optical network units (“ONUs”). Optical signals of a certain group of wavelengths are transmitted (upstream signal) from the optical device to the ONUs. Additionally, optical signals of a certain group of wavelengths, which may be different from the group of wavelengths of the transmitted optical signals, are transmitted (downstream signal) from the ONUs to the optical device.
In order for the optical device to transmit or receive at a desired wavelength from among the group of wavelengths, the device must be tuned. For a upstream signal, the signal generator (e.g., laser) of the device may be tunable to provide what is effectively a “pre-filtered” signal. Alternatively, the laser may transmit an optical signal to a destination ONU with an optical transmission made up of multiple wavelengths (with minimal or no filtering applied), and the optical signal can be filtered from the transmission at the destination ONU. Similarly, the ONU may transmit a multiple-wavelength optical signal to the optical device, and the signal can be filtered at the optical device. In order to filter such a signal, a tunable optical filter may be provided. To accommodate for transmitting and receiving both “pre-filtered” and non-“pre-filtered” optical signals, the optical device may include either or both of a laser and optical filter that can be tuned to a frequency that corresponds to the desired wavelength. Conventionally, this tuning is performed by changing the temperature of the laser / optical filter. Thus, tuning at the optical device may be performed using a temperature dependent tunable element.
As passive optical networks (PONs) become increasingly faster, it becomes increasingly more important for the tunable element to be tuned at a high speed. At the same time, while it is important to be able to tune the tunable element at a high speed, it is also necessary for the tuning to be precise and accurate in order to minimize attenuation of the desired downstream or upstream signal (e.g., as it passes through the cable or filter). In other words, it is desirable to both rapidly and precisely change a temperature of the tunable element. It is further desirable for the optical device to have a relatively compact size, and to minimize the cost of the temperature control components, as well as the cost of their installation.
Additionally, as the spacing between optical channels in a PON becomes smaller and smaller, and as the bandwidth of each channel becomes increasingly narrowing, it becomes increasingly more important for the tunable element to be locked at a given frequency/bandwidth. Locking the tunable element increases the precision and accuracy, and further minimizes attenuation, of the desired downstream or upstream signal (e.g., as it passes through the cable or filter). In other words, while it is desired to both rapidly and precisely change a temperature of the tunable element, such precision in tunability should not come at the expense of being able to precisely lock the tuned optical signal in place.